Question

Which enzyme will be produced in a cell in which there is nonsense mutation in the lac y gene?
A. β-galactosidase
B. Lactose permease
C. Transacetylase
D. Lactose permease and transacetylase

Answer 1

Hint:-Francis Jacob and Jacques Monad while working at Pasteur Institute, Paris, proposed that metabolic pathways are regulated as a unit. They studied the change in the nature of enzymes of E.coli when provided with different kinds of sugars.

Complete answer:
They found that when lactose is added to the culture of E. coli, it produces three enzymes necessary to break down the lactose into galactose.
These newly synthesized enzymes are:
(i) β-galactosidase
(ii) permease and
(iii) transacetylase.
According to them the synthesis of three enzymes is controlled by a long segment called operon which is divisible into an operator site O and three structural genes Z, Y, and A. The operator site with the help of the structural gene regulates the action of regulator gene (i).
The gene Z produces enzyme β-galactosidase for splitting of lactose/galactosidase into glucose and galactose. The gene Y produces enzyme permease that helps in the entry of lactose and gene A which produces enzyme galactosidase acetylase and transacetylase.
Mutations are defined as sudden changes. Nonsense mutation is the type of mutation in which there is a substitution of a single codon resulting in formation of a stop codon which terminates the polypeptide synthesis. The gene Z is transcribed before Lac Y and A gene so due to nonsense mutation in the lac Y gene, a stop codon will appear in the sequence after lac Z gene. Thus β-galactosidase will be produced but no other enzyme will be produced.

So the correct option is A. β-galactosidase

Note:- Lac-operon is an example of inducible operon as a few molecules of lactose are added into the cell by the action of the enzyme permease. These molecules when added, convert into an active form of lactose which binds to the repressor and thus the repressor does not bind to the operator.